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Plants grow within a complex web of species that interact with each other and with the plant 1 – 10 . These interactions are governed by a wide repertoire of chemical signals, and the resulting chemical landscape of the rhizosphere can strongly affect root health and development 7 – 9 , 11 – 18 . Here, to understand how interactions between microorganisms influence root growth in Arabidopsis , we established a model system for interactions between plants, microorganisms and the environment. We inoculated seedlings with a 185-member bacterial synthetic community, manipulated the abiotic environment and measured bacterial colonization of the plant. This enabled us to classify the synthetic community into four modules of co-occurring strains. We deconstructed the synthetic community on the basis of these modules, and identified interactions between microorganisms that determine root phenotype. These interactions primarily involve a single bacterial genus ( Variovorax ), which completely reverses the severe inhibition of root growth that is induced by a wide diversity of bacterial strains as well as by the entire 185-member community. We demonstrate that Variovorax manipulates plant hormone levels to balance the effects of our ecologically realistic synthetic root community on root growth. We identify an auxin-degradation operon that is conserved in all available genomes of Variovorax and is necessary and sufficient for the reversion of root growth inhibition. Therefore, metabolic signal interference shapes bacteria–plant communication networks and is essential for maintaining the stereotypic developmental programme of the root. Optimizing the feedbacks that shape chemical interaction networks in the rhizosphere provides a promising ecological strategy for developing more resilient and productive crops. Experiments using an ecologically realistic 185-member bacterial synthetic community in the root system of Arabidopsis reveal that Variovorax bacteria can influence plant hormone levels to reverse the inhibitory effect of the community on root growth.

Lytic bacteriophages (‘phages’) can limit bacterial densities and shape community structure, either directly through lysis or indirectly through costs to resistance. However, phages have also been reported to have no, and in some cases even positive, effects on host densities. Here, we investigate the mechanisms behind an increase in host density in Variovorax sp. populations following a fixation of resistance that was maintained after phage extinction. Our results demonstrate that the density increase was a genetic trait coinciding with resistance emergence. Growth curves showed that phage resistance shifted population growth curves such that density was higher in the death phase. This density-increasing effect of resistance had important implications for community structure with phage-resistant Variovorax decreasing the density of a conspecific. That resistance to lytic phage can increase host densities has implications for wider ecology and phage therapy, where lytic phages are presumed to have negative effects on their hosts.

Intraterrestrial waters harbor microbial communities being extensively studied to understand microbial processes underlying subsurface ecosystem functioning. This paper provides the results of an investigation on the microbiomes of unique, subsurface sulfidic waters associated with Upper Jurassic, Cretaceous, and Miocene sediments. We used high-throughput 16S rDNA amplicon sequencing to reveal the structure of bacterial and archaeal communities in water samples differing in sulfide content (20–960 mg/dm3), salinity (1.3–3.2%), and depth of extraction (60–660 m below ground level). Composition of the bacterial communities strongly varied across the samples; however, the bacteria participating in the sulfur cycle were common in all sulfidic waters. The shallowest borehole water (60 m bgl) was dominated by sulfur-oxidizing Epsilonproteobacteria (Sulfurimonas, Sulfurovum). In the waters collected from greater depths (148–300 m bgl), the prevalence of Betaproteobacteria (Comamonadaceae) and sulfate/ sulfur-reducing Deltaproteobacteria (Desulfopila, Desulfomicrobium, MSBL7) was observed. Sulfate reducers (members of Clostridia: Candidatus Desulforudis) were the most abundant bacteria in the deepest borehole water (660 m bgl). Out of 850 bacterial OTUs, only one, affiliated with the Comamonadaceae family, was found abundant (> 1% of total bacterial sequences) in all samples. Contribution of Archaea to the whole microbial communities was lower than 0.5%. Archaeal communities did not differ across the samples and they consisted of Halobacteriaceae. Out of 372 archaeal OTUs, five, belonging to the four genera Natronomonas, Halorubrum, Halobellus, and Halorhabdus, were the most numerous.

Acidovorax citrulli, the causal pathogen of bacterial fruit blotch of cucurbits, relies on a functional type III secretion system (T3SS) for pathogenicity. Two‐component systems (TCSs) are primary signal transduction mechanisms for bacteria to detect and adapt to various environmental conditions. However, the role of TCS on regulating T3SS and other virulence factors in response to environmental stimuli is still poorly understood in A. citrulli. Here, we report the identification of a conserved TCS, OmpR/EnvZ, involved in hypersensitive response (HR) induction in Nicotiana benthamiana by screening a transposon‐insertion library in the group II strain xjL12 of A. citrulli. Transcription analysis confirmed that OmpRAc/EnvZAc was upregulated in response to elevated osmotic pressure, low and high pH conditions, and host environment. Deletions of envZAc, ompRAc, or both envZAc and ompRAc in A. citrulli attenuated virulence to melon seedlings and mature leaf tissues, and delayed HR in N. benthamiana. OmpRAc was activated by EnvZAc and directly bound to the promoter region of hrpG, a major regulator of T3SS. This binding activated hrpG transcription and promoted T3SS assembly in T3SS‐inducing medium, XVM2. Additionally, the OmpRAc/EnvZAc mutants of A. citrulli displayed reduced swimming motility due to impaired flagella formation, but also had enhanced biofilm formation and exopolysaccharide production. OmpRAc/EnvZAc regulation of these virulence factors in A. citrulli depended on its own conserved phosphorylation sites. This work illuminates a signalling pathway for regulating the T3SS and provides insights into the OmpR/EnvZ‐mediated virulence regulatory network in A. citrulli. Acidovorax citrulli OmpR/EnvZ responds to extracellular pH and osmolarity, controls the T3SS by binding to the hrpG promoter, and is involved in regulating flagellar biogenesis, biofilm, and exopolysaccharide production.

The climate-active gas isoprene (2-methyl-1,3-butadiene) is released to the atmosphere in huge quantities, almost equaling that of methane, yet we know little about the biological cycling of isoprene in the environment. Although bacteria capable of growth on isoprene as the sole source of carbon and energy have previously been isolated from soils and sediments, no microbiological studies have targeted the major source of isoprene and examined the phyllosphere of isoprene-emitting trees for the presence of degraders of this abundant carbon source. Here, we identified isoprene-degrading bacteria in poplar tree-derived microcosms by DNA stable isotope probing. The genomes of isoprene-degrading taxa were reconstructed, putative isoprene metabolic genes were identified, and isoprene-related gene transcription was analyzed by shotgun metagenomics and metatranscriptomics. Gram-positive bacteria of the genus Rhodococcus proved to be the dominant isoprene degraders, as previously found in soil. However, a wider diversity of isoprene utilizers was also revealed, notably Variovorax, a genus not previously associated with this trait. This finding was confirmed by expression of the isoprene monooxygenase from Variovorax in a heterologous host. A Variovorax strain that could grow on isoprene as the sole carbon and energy source was isolated. Analysis of its genome confirmed that it contained isoprene metabolic genes with an identical layout and high similarity to those identified by DNA-stable isotope probing and metagenomics. This study provides evidence of a wide diversity of isoprene-degrading bacteria in the isoprene-emitting tree phyllosphere and greatly enhances our understanding of the biodegradation of this important metabolite and climate-active gas.

Anodic microbial communities in acetate-fed microbial fuel cells (MFCs) were analyzed using stable-isotope probing of 16S rRNA genes followed by denaturing gradient gel electrophoresis. The results revealed that Geobacter sulfurreducens and Hydrogenophaga sp. predominated in the anodic biofilm. Although the predominance of Geobacter sp. as acetoclastic exoelectrogens in acetate-fed MFC systems has been often reported, the ecophysiological role of Hydrogenophaga sp. is unknown. Therefore, we isolated and characterized a bacterium closely related to Hydrogenophaga sp. (designated strain AR20). The newly isolated strain AR20 could use molecular hydrogen (H 2 ), but not acetate, with carbon electrode as the electron acceptor, indicating that the strain AR20 was a hydrogenotrophic exoelectrogen. This evidence raises a hypothesis that acetate was oxidized by G. sulfurreducens in syntrophic cooperation with the strain AR20 as a hydrogen-consuming partner in the acetate-fed MFC. To prove this hypothesis, G. sulfurreducens strain PCA was cocultivated with the strain AR20 in the acetate-fed MFC without any dissolved electron acceptors. In the coculture MFC of G. sulfurreducens and strain AR20, current generation and acetate degradation were the highest, and the growth of strain AR20 was observed. No current generation, acetate degradation and cell growth occurred in the strain AR20 pure culture MFC. These results show for the first time that G. sulfurreducens can oxidize acetate in syntrophic cooperation with the isolated Hydrogenophaga sp. strain AR20, with electrode as the electron acceptor.

Eight bacterial isolates closely related to Diaphorobacter sp. were isolated from activated biomass surviving on wastewater laden with dyes and nitro-substituted chemicals and were identified by 16S rDNA sequence analysis. The isolates showed sequence similarity of 99-100% to other Diaphorobacter strains such as ZY 2006b, F2, NA5, PCA039, D. nitroreducens KSP4, and KSP3 and 98-99% sequence homology to D. nitroreducens NA10B (type strain JCM 11421). Neighbor-joining tree revealed that all the eight strains formed tight cluster together and also showed close clustering with other Diaphorobacter strains. Isolates demonstrated the ability to perform simultaneous nitrification and denitrification under aerobic conditions. Strains HPC 805, 815, 821, and 856 gave highest chemical oxygen demand removal (85-93%) and ammonia removal (92-96%), which correlated well with higher growth rates of the cultures. Simultaneously, complete removal of nitrate supplied in the medium in presence of ammonium and acetate (electron donor) was observed in addition to aerobic nitrite release from ammonium. Thus, the above strains showed ability to perform partial nitrification followed by further aerobic removal of common intermediate nitrite, which indicated their potential application in treatment systems for treatment of high-nitrogen-containing wastewaters.

Plant growth-promoting rhizobacteria (PGPR) of the genus Variovorax facilitate plant growth through beneficial microbe-plant interplay. Unlike most PGPRs, Variovorax boronicumulans CGMCC 4969 utilizes indole-3-acetonitrile as a precursor to generate indole-3-acetic acid (IAA), which is subsequently metabolized by itself. In this study, IAA enhanced the growth of V. boronicumulans CGMCC 4969 in minimal salt medium (MSM), whereas it inhibited bacterial growth when glucose was added to the MSM broth. IAA was rapidly degraded within 12 h in MSM broth despite glucose appeared or not. Notably, in LB broth, the cell growth was significantly inhibited by IAA concentration beyond 1 mmol/L, while the IAA degradation capability of CGMCC 4969 was significantly increased following exposure to IAA-dosed LB medium. V. boronicumulans CGMCC 4969 degraded IAA to yield a new intermediate 3-hydroxy-anthranilate. An iad gene cluster was identified in V. boronicumulans CGMCC 4969, and co-expression of the iadD and iadE genes endows Escherichia coli with the capacity to degrade IAA. This degradation efficiency is augmented when the iadC gene is expressed simultaneously. Subsequent proteomics and bioinformatics analyses highlighted that the addition of IAA induced a significant up-regulation of ABC transporter proteins, in particular IadK3 and IadK2. Interestingly, there was also a significant increase in protein expression associated with group-sensing metabolism. Collectively, this research helps our understanding of the intricate regulatory mechanisms of IAA within Variovorax own metabolism and expands our knowledge of its complex role in plant-microbe interactions. KEY POINTS: The iad gene cluster degraded IAA to a previously uncharacterized intermediate. Adding IAA during the cell culture period enhances IAA-degrading activity. Proteomics defined the adaptive response to IAA.

Springtails are important members of the soil fauna and play a key role in plant litter decomposition, for example through stimulation of the microbial activity. However, their interaction with soil microorganisms remains poorly understood and it is unclear which microorganisms are associated to the springtail (endo) microbiota. Therefore, we assessed the structure of the microbiota of the springtail Orchesella cincta (L.) using 16S rRNA gene amplicon sequencing. Individuals were sampled across sites in the field and the microbiota and in particular the endomicrobiota were investigated. The microbiota was dominated by the families of Rickettsiaceae , Enterobacteriaceae and Comamonadaceae and at the genus level the most abundant genera included Rickettsia , Chryseobacterium , Pseudomonas , and Stenotrophomonas . Microbial communities were distinct for the interior of the springtails for measures of community diversity and exhibited structure according to collection sites. Functional analysis of the springtail bacterial community suggests that abundant members of the microbiota may be associated with metabolism including decomposition processes. Together these results add to the understanding of the microbiota of springtails and interaction with soil microorganisms including their putative functional roles.

An increasing number of evidences indicate microbes are implicated in human physiological mechanisms, including complicated disease pathology. Some microbes have been demonstrated to be associated with diverse important human diseases or disorders. Through investigating these disease-related microbes, we can obtain a better understanding of human disease mechanisms for advancing medical scientific progress in terms of disease diagnosis, treatment, prevention, prognosis and drug discovery. Based on the known microbe-disease association network, we developed a semi-supervised computational model of L aplacian R egularized L east S quares for H uman M icrobe– D isease A ssociation (LRLSHMDA) by introducing Gaussian interaction profile kernel similarity calculation and Laplacian regularized least squares classifier. LRLSHMDA reached the reliable AUCs of 0.8909 and 0.7657 based on the global and local leave-one-out cross validations, respectively. In the framework of 5-fold cross validation, average AUC value of 0.8794 +/−0.0029 further demonstrated its promising prediction ability. In case studies, 9, 9 and 8 of top-10 predicted microbes have been manually certified to be associated with asthma, colorectal carcinoma and chronic obstructive pulmonary disease by published literature evidence. Our proposed model achieves better prediction performance relative to the previous model. We expect that LRLSHMDA could offer insights into identifying more promising human microbe-disease associations in the future.